<!DOCTYPE html>
 	<html lang="en">
 	<head>
 	    <meta charset="UTF-8">
 	    <title>Document</title>
 	     <style>
 	    div{ width:50px; height: 50px; border:1px solid;}
 	</style>
 	</head>
 	<body>
 	     <div></div>
 	     <p> <button id="btn">change</button> </p>
 	     <script>
 	   
 	    var btn = document.getElementById('btn');
 	    var div = document.getElementsByTagName('div')[0];
 	    btn.onclick = function () {
 	        div.style.border = 'double red';
 	    }
 	     </script>
 	</body>
 	</html>
